2. Field of the Invention
This invention relates to an intravenous infusion pump set including an air eliminating filter with an in-line hydrophilic membrane to trap air in a medical infusion device line and a hydrophobic membrane to vent the trapped air. An anti-siphon valve with a positive crack pressure is situated in the line between the air eliminating filter and the infusion site.
2. Description of the Prior Art
The use of air eliminating filters in intravenous infusion pump sets is often required with the use of large volume pharmaceutical product bags due to the difficulty of purging all the air from the system. Furthermore, high infusion rate systems have an increased severity upon air embolism and therefore particularly require effective air elimination.
Air eliminating filters come in two different designs. The first design uses a standard hydrophilic membrane to prevent the passage of air bubbles and other gases through the apparatus. The pore size of a hydrophilic membrane is typically 0.2 micrometers. Hydrophilic membranes, when wetted, prevent the passage of air and gases therethrough up to the bubble point pressure but allow the passage of liquids therethrough. An intravenous set with an in-line hydrophilic membrane therefore allows a user to purge air from the intravenous set until liquid wets the filter and then no air will pass. The filter then collects air or gases on the upstream side of the membrane. Therefore, air accumulates during operation of the first design of filter thereby restricting flow of the liquid as air accumulates. The restriction increases until the flow stops when the membrane is covered by air thereby causing a complete occlusion.
The second design of filter used for air elimination includes an in-line hydrophilic membrane similar to the first design and additionally includes a vent covered with a hydrophobic membrane on the upstream side of the hydrophilic membrane housing thereby allowing the escape of the trapped gases. The hydrophobic membrane will allow only air (or gases) to pass therethrough thereby removing the trapped air from the filter. Therefore, this second design of filter will continue to pass the liquid from a liquid/air bubble mixture whereas the first design allows the bubbles to accumulate and eventually occlude the passage of liquid.
The use of this second design of filter which includes an in-line hydrophilic membrane and a hydrophobic membrane placed upstream of the hydrophilic membrane is well-known in the prior art. An example of such a filter is disclosed in U.S. Pat. No. 3,650,093 to Rosenberg.
However, as such prior art apparatus heretofore uses no anti-siphon or positive crack pressure valve means downstream of the filter, two problems will occur. Firstly, under certain common combinations of flow rate and resistance, lowering the infusion site relative to the filter will produce a negative pressure in the set resulting in air being admitted through the hydrophobic membrane. While this air can never pass through the hydrophilic membrane, its entry results in a bolus of pharmaceutical product to the patient. Secondly, if air exists in the filter either by incomplete purging, by entry from the pharmaceutical product container, or by prior admission through the hydrophobic membrane, elevation of the infusion site (such as may occur during ambulatory circumstances) will expel this air through the hydrophobic membrane. Retrograde blood flow from the patient will occur equal to the volume of air expelled.
An intravenous set which includes an air eliminating filter placed between the pharmaceutical product bag and the pump is similarly deficient. This location works well if the filter is always below the fluid level in the reservoir, as is common practice. However, if the pharmaceutical product bag of such a set is inadvertently or otherwise placed below the pump, then the air eliminating filter may draw air into itself and result in an error in flow rate or total occlusion. The air will not pass the hydrophilic membrane, but can cause air locking of the filter thereby preventing liquid from being drawn from the pharmaceutical product reservoir.
An intravenous set which includes an air eliminating filter between the pump and the patient is also deficient. If the patient is placed below the filter of such a set, the filter may draw in air from the atmosphere, particularly with low flow rates, thereby causing an additional error in flow rate. Further, if the patient is then moved from below the filter to above the filter, backflow of fluid from the patient to the filter will occur causing the intravenous set to fill with blood from the patient. Moreover, if the patient is below the pharmaceutical product bag and the set is not properly in the pump and therefore not occluded, a free flow, or siphoning, of solution into the patient will occur.